Metal containers for aerosol, for example, are largely grouped into three-piece cans and two-piece cans. The three-piece can has a structure of a can body prepared by joining edges of cylindrically-shaped rectangular sheet, and a can bottom and a can lid (dome top) attached to the respective ends of the can body. The name of the three-piece can comes from the three structural elements of the members. When the three-piece can is used for spraying, the dome top is further equipped with a mounting cap having an ejection valve, (when the term “mounting cap” is used hereinafter, the cap is equipped with the ejection valve). Therefore, when the mounting cap is counted, the number of structural members becomes four. Since, however, beverage cans and food cans which have a can body joining the edges of cylindrically-formed metal sheet are usually called the “three-piece cans,” we also name the cans having the above structure as the “three-piece cans.”
The two-piece can is fabricated by reducing the diameter of the can body, formed into a shape of cylinder integrated with bottom, at the opening end side, and by attaching the mounting cap to the opening. For the spraying use, the two-piece can has only one structural member, or the can body, except for the mounting cap. In some cases the can body is called the one-piece can or monoblock can. Since, however, for beverage cans and food cans, the can having the can body formed into a shape of cylinder integrated with bottom is widely called the “two-piece,” we also name the can body having the structure as the “two-piece can.” The two-piece can has a seamless can body, and is treated by diametral reduction in a range from the can body toward the mounting cap in smooth and continuous shape. Therefore, compared with the three-piece can, the two-piece can has superior beautifulness in appearance. As a result, the two-piece can is widely adopted by the users emphasizing the appearance of package for appealing the product of aromatic substance, antiperspirant, and hair conditioner.
The base material for a three-piece can and a two-piece can is normally steel sheet for the three-piece can, and the base material for a two-piece can is normally aluminum. The two-piece can adopts aluminum because aluminum is softer than steel sheet so that aluminum is relatively easily worked into the shape of aerosol can specified in Federation of European Aerosol Association Standard Nos. 215, 219 and 220 through the forming of the can body in a shape of a cylinder integrated with a bottom by impact-forming, drawing-redrawing forming, drawing-redrawing-ironing forming, and the like, followed by diametral reduction of the opening end. A more important point is that, since aluminum has higher corrosion resistance than that of steel sheet, the corrosion resistance to the contents and the rust-generation on the outer face raise little problem under exposure of an aerosol can to a humid environment. To the contrary, steel sheet has high strength and is inexpensive. Consequently, if steel sheet is applied to an aerosol can which needs high pressure strength, the can sheet thickness can be reduced while assuring sufficient can strength, thereby decreasing the base material cost. From the above consideration, there are studies of fabrication of two-piece cans using a high steel sheet having increased corrosion resistance.
For example, the methods for fabricating a two-piece aerosol can using a steel sheet having increased corrosion resistance are disclosed in Published Japanese Patent Translation of PCT Application 2003-500306 for the method to increase the corrosion resistance of the steel sheet itself, in Japanese Patent Laid-Open No. 63-168238 for the method in which the surface of steel sheet is laminated by a metal having high corrosion resistance, in Japanese Patent Laid-Open No. 9-39975 for the method in which the surface of steel sheet is laminated by a coating film, and in Japanese Patent Laid-Open Nos. 1-228567 and 10-24973 for the method in which the surface of steel sheet is laminated by a film.
Published Japanese Patent Translation of PCT Application 2003-500306 for the method of increasing the corrosion resistance of steel sheet discloses a technology to adopt a highly anticorrosive stainless steel as the steel sheet. Stainless steel is, however, an expensive one, though the corrosion resistance is high, thus the method increases the cost.
Japanese Patent Laid-Open No. 63-168238, in which the surface of steel sheet is coated with a metal having high corrosion resistance, discloses a technology which uses a steel sheet coated with aluminum, thereby avoiding the rust generation at the can bottom of the two-piece aerosol can formed by drawing and ironing. The method may avoid rust generation at the can bottom where the degree of working is small. Since, however, the can body which is subjected to drawing and ironing suffers damage of aluminum coating, the rust generation may occur.
Japanese Patent Laid-Open No. 9-39975, which is the method of coating the surface of steel sheet with a lacquer layer, discloses a technology relating to the inside-lacquered metal container having a cured polyamide-imide-based lacquer layer. Although the technology is described to be able to use a steel sheet as the base material for a two-piece aerosol can, the Examples relating to the steel sheet in the description are only for three-piece cans subjected to a small degree of working, and there is no satisfactory description about the corrosion resistance for the steel sheet which is worked into a two-piece can subjected to high degree of working, and the effect is not known. In addition, the specification describes that the technology can be applied either to the formed can body or to the metal sheet before working. However, for the case of applying the technology to an aluminum-made two-piece can, given in the Examples, the lacquer coating after forming the can body is described, but there is no detail example of the case that the metal sheet before forming is coated with lacquer layer. To this point, according to the investigations conducted by us, when a steel sheet coated by a thermally cured lacquer layer was worked into a two-piece aerosol can subjected to a high degree of working, the lacquer layer was damaged by the working, thus failing to attain sufficient corrosion resistance.
In view of corrosion resistance, the method of laminating the surface of steel sheet by a film is a promising one. Japanese Patent Laid-Open No. 1-228567 discloses a technology of fabricating an aerosol can using also a steel sheet laminated by a biaxially oriented film of poly-ethylene terephthalate. According to the technology, since the drawn can body is laminated by a not-damaged laminate film, the corrosion resistance is strong. However, the corrosion resistance on the can body obtained by the technology is maintained only for the can at a small working degree, receiving no diametral reduction at the opening end of the can body, as given in the Examples, and there is no consideration of the corrosion resistance on applying to the can worked into the shape of aerosol can, specified in Federation of European Aerosol Association Standard Nos. 215, 219 and 220.
Japanese Patent Laid-Open No. 10-24973 discloses a technology relating to an aerosol can which is fabricated by drawing a steel sheet laminated by a composite film composed of a polypropylene resin layer laminated on both sides of a vinylidene chloride resin layer via an acid-modified polyolefin resin, respectively. Since the technology uses a steel sheet laminated by a film, the can body having high corrosion resistance is expected to be obtained. For the method of drawing, however, a detailed description of working method is not given, only describing in the Examples that an aerosol can having a shape of 45 mm in diameter and 120 mm in height was obtained. In particular, no disclosure is given on the corrosion resistance after diametral reduction at the opening end of the can body.
As of those technologies, the use of a laminated steel sheet, having the laminate of an organic resin film, as the base material of the can, as in Japanese Patent Laid-Open Nos. 1-228567 and 10-24973, is advantageous in terms of corrosion resistance. Our investigations, however, revealed that sole application of laminated steel sheet of the conventional technologies is not satisfactory to obtain the two-piece aerosol can.
Specifically, forming of a two-piece aerosol can having a standard shape specified in Federation of European Aerosol Association Standard Nos. 215, 219 and 220 needs to form a laminated steel sheet at a very high degree of working, which induces separation on laminated steel sheet during the forming step, resulting in failing to maintain sufficient corrosion resistance.
Thus, it could be advantageous to provide a two-piece can having sufficient can strength and high corrosion resistance using a laminated steel sheet which is high in strength, relatively inexpensive, and high in corrosion resistance, and to provide a forming method which easily manufactures the two-piece can without generating separation of laminate film.
Through our studies, we found the following. First, the can strength of the two-piece can becomes sufficient at a low cost by using a laminated steel sheet high in strength, relatively inexpensive, and high in corrosion resistance as the base material of the two-piece aerosol can. Regarding the prevention of deterioration of corrosion resistance, occurring accompanied with the separation of laminate film and the like during working of steel sheet as the base material for the two-piece can, we found that it is effective to form the opening end in a state of being clamped between a drawing die and a blank holder, without generating sliding. In concrete terms, a means to avoid damages on the laminated steel sheet at the opening end of the can body is to form a tapered part extending outward from the can body in the radial direction thereof at the tip of the opening of the can body, before the final drawing step, followed by applying diametral reduction to the tapered part to the diameter of the can body.
Thus, selected aspects of our methods, steel sheets and two-piece cans include:                1. A method for forming two-piece can using a laminated steel sheet as a base material, having the steps of: blanking the laminated steel sheet; forming a can body in a shape of cylinder integrated with bottom by applying repeated several cycles of drawing to a circular blank obtained by blanking the laminated steel sheet; forming a dome protruding inside of the can at the can bottom; trimming the opening of the can body; applying diametral reduction to the opening of the can body after trimming to a diameter smaller than the diameter of the can body, while satisfying the formulae (1) and (2); forming a bead at the opening end of the can body after the diametral reduction, thus obtaining the two-piece can, wherein the above working steps include the step of applying heat treatment to the laminated steel sheet during the working steps, and wherein the heat treatment step includes the step of heating the laminated steel sheet to a temperature in a range from the glass transition point of the film of the laminated steel sheet to the melting point thereof +30° C., followed by applying rapid cooling to the laminated steel sheet to a temperature of the glass transition point or below within 30 seconds after the heating,1.5≦h/(R−r)  (1)d/R≦0.25  (2)        where, h is the height from the can bottom to the tip of the opening, R is the circular blank positional radius, r is the radius of the can bottom, and d is the radius of the tip of the opening.        2. A method for forming two-piece can using a laminated steel sheet as a base material, having the steps of: blanking the laminated steel sheet; forming a can body in a shape of cylinder integrated with bottom by applying repeated several cycles of drawing to a circular blank of the laminated steel sheet obtained by blanking the laminated steel sheet; forming a dome protruding inside of the can at the can bottom; trimming the opening of the can body; applying diametral reduction to the opening of the can body after trimming to a diameter smaller than the diameter of the can body, while satisfying the formulae (1) and (2); forming a bead at the opening end of the can body, thus obtaining the two-piece can, wherein the above working steps include the step of applying heat treatment to the laminated steel sheet during the working steps, and wherein the heat treatment step includes the step of heating the laminated steel sheet to a temperature in a range from the glass transition point of the film of the laminated steel sheet to the melting point thereof +30° C., followed by applying rapid cooling to the laminated steel sheet to a temperature of the glass transition point or below within 30 seconds after the heating, at a stage that the degree of working is in a range of 0.2≦d/R≦0.5 and of 1.5≦h/(R−r)≦2.5,1.5≦h/(R−r)  (1)d/R≦0.25  (2)        where, h is the height from the can bottom to the tip of the opening, R is the circular blank positioning radius, r is the radius of the can bottom, and d is the radius of the tip of the opening.        3. The method for forming two-piece can according to 1 or 2, further having the step of trimming the opening of the can body to form a new opening end, after the step of applying diametral reduction to the opening of the can body.        4. The method for forming two-piece can according to 1 or 2, wherein the laminated steel sheet is a steel sheet laminated by a polyester resin.        5. The method for forming two-piece can according to 4, wherein the polyester resin is prepared by polycondensation of a dicarboxylic acid component and a diol component, wherein the dicarboxylic acid component contains terephthalic acid as the main ingredient, and the diol component contains at least one of ethylene glycol and butylene glycol as the main ingredient.        6. The method for forming two-piece can according to 5, wherein the organic resin as the laminate of the laminated steel sheet contains a polyester resin as the main phase, and a resin which is incompatible and has 5° C. or lower Tg as the sub-phase. (Note: The glass transition is a phenomenon that a polymer substance changes the glassy hard state to a rubber-like state when it is heated. The temperature of occurrence of the glass transition is called the “glass transition point (Tg)”).        7. The method for forming two-piece can according to 6, wherein the resin existing as the sub-phase is a resin selected from the group consisting of: polyethylene, an acid modification thereof, an ionomer thereof, polypropylene, an acid modification thereof, and an ionomer thereof.        8. A two-piece can being formed by the forming method according to any of 1 to 7.        9. A laminated steel sheet for two-piece can, being used for the method for forming two-piece can according to any of 1 to 7. The laminated steel sheet for two-piece can is a laminated steel sheet having the laminate of an organic resin film.        10. A method for forming two-piece can using a laminated steel sheet as a base material, having the steps of: blanking the laminated steel sheet; forming a can body in a shape of cylinder integrated with bottom by applying repeated several cycles of drawing to a circular blank of the laminated steel sheet, obtained by blanking the laminated steel sheet; forming a tapered part extending outward from the can body in the radial direction thereof at the tip of the opening of the can body; applying diametral reduction to the tapered part to the diameter of the can body; and applying diametral reduction to the opening side of the can body to a diameter smaller than the diameter of the can body, while satisfying the formulae (1) and (2),1.5≦h/(R−r)  (1)d/R≦0.25  (2)        where, h is the height from the can bottom to the tip of the opening, R is the circular blank positioning radius, r is the radius of the can bottom, and d is the radius of the tip of the opening.        11. The method for forming two-piece can according to 10, further having the step of trimming to form a new opening end at the tapered part in at least one of the stages of: before the step of applying diametral reduction to the tapered part; and after the step of applying diametral reduction to the tapered part.        12. The method for forming two-piece can according to 10 or 11, further having the step of trimming to form a new opening end at the opening side of the can body after the step of applying diametral reduction to the opening side of the can body.        13. The method for forming two-piece can according to 10, wherein the laminated steel sheet is a steel sheet laminated by a polyester resin.        14. The method for forming two-piece can according to 13, wherein the polyester resin is prepared by polycondensation of a dicarboxylic acid component and a diol component, wherein the dicarboxylic acid component contains terephthalic acid as the main ingredient, and the diol component contains ethylene glycol and/or butylene glycol as the main ingredient.        15. The method for forming two-piece can according to 14, wherein the organic resin as the laminate of the laminated steel sheet contains a polyester resin as the main phase, and a resin which is incompatible and has 5° C. or lower Tg as the sub-phase.        16. The method for forming two-piece can according to 15, wherein the resin existing as the sub-phase is a resin selected from the group consisting of: polyethylene, an acid modification thereof, an ionomer thereof, polypropylene, an acid modification thereof, and an ionomer thereof.        17. A two-piece can being formed by the forming method according to any of 10 to 16.        18. A laminated steel sheet having the laminate of an organic resin film, being used for the method for forming two-piece can according to any of 10 to 16.        19. The method according to any of 1, 2, and 10, wherein the forming method of two-piece can made of the laminated steel sheet includes ironing adding to the deep drawing.        20. The method according to any of 1, 2, and 10, wherein the number of drawing cycles for forming the two-piece can of the laminated steel sheet is preferably 10 or less.        21. The method according to any of 1, 2, and 10, wherein the drawing rate on forming the two-piece can of the laminated steel sheet is preferably 0.4 or more for the first drawing cycle on the circular blank, and 0.5 or more for succeeding drawing (re-drawing) cycles.        22. The method according to any of 1, 2, and 10, wherein, on applying drawing to the laminated steel sheet, the average sheet thickness change rate is preferably in a range of 0.5<t/t0<1.5, where t is the average sheet thickness over the entire sheet thickness of side wall of the can body, and to is the original sheet thickness.        23. The method according to any of 1, 2, and 10, wherein the degree of working on applying diametral reduction to the opening end of the two-piece can of the laminated steel sheet is preferably in a range of d/r>0.3 and, more preferably, in a range of d/r>0.4, where r is the radius of can body, and d is the radius of opening end after the diametral reduction.        24. The method according to any of 1, 2, and 10, wherein the steel sheet becoming the substrate of the laminated steel sheet contains at least one of the following:                    (1) a steel sheet of a low carbon steel of approximate range from 0.01 to 0.10% C, being prepared by recrystallization annealing by box annealing;            (2) a steel sheet of a low carbon steel of approximate range from 0.01 to 0.10% C, being prepared by recrystallization annealing by continuous annealing;            (3) a steel sheet of a low carbon steel of approximate range from 0.01 to 0.10% C, being prepared by recrystallization annealing by continuous annealing, followed by over-aging;            (4) a steel sheet of a low carbon steel of approximate range from 0.01 to 0.10% C, being prepared by recrystallization annealing by box annealing or continuous annealing, followed by secondary cold-rolling (cold-rolling after annealing); and            (5) an IF steel (interstitial free steel) of a very low carbon steel of approximate range of 0.003% or less C with the addition of a solid-solution C-fixing element, prepared by recrystallization annealing by continuous annealing.                        25. The method according to any of 1, 2, and 10, wherein the method for applying diametral reduction to the laminated steel sheet is at least one of the die-curling method and the spinning method.        